The RS-485, as a traditional industrial communication bus, uses balanced transmission and differential reception, and has advantages such as a strong capability of common mode interference suppression, a long transmission distance, availability in multi-point interconnection, and low cost etc., and therefore is widely used in various industrial control fields. Due to a poor industrial field environment, the RS-485 port is generally required to have a protection circuitry to protect the device against influences of lightning, surge and static electricity.
In the protection circuit of the RS-485 port, according to the actual application scenarios, a current limiting device and a Transient Voltage Suppressor (TVS) are mostly used to simply perform a process of protection against surge and static, which has a low protection level. In some cases, two lines of a first level at an entrance of the RS-485 port are connected to the protection ground via a gas discharge tube respectively, and a Positive Temperature Coefficient (PTC) resistor is connected in series respectively on the two lines; and two lines of a second level are connected to the protection ground respectively via a bidirectional or uni-directional TVS, and then there is a bidirectional TVS between lines, which is a strong protection structure.
In practical applications, due to differences in the ground connection environment for the protection ground in the field, it results in a voltage difference between the protection ground and the actual ground. Such circuit of a strong protection structure is not necessarily capable of suppressing lightning and surge effectively, and the breakdown voltage from the signal lines of the second level to the TVS is very low, and is generally only a few volts. Therefore, it is prone for the voltage of the protection ground to rise and break down, and thus a condition that the RS-485 communication port is damaged occurs.